The present invention relates to a method and system of monitoring and controlling deflection of a wafer during a plasma etching process.
When processing wafer substrates for use in electronic and optoelectronic applications there is a need to ensure the wafer surfaces are very smooth and clean. Once the bulk wafer has been prepared various grinding, polishing and etching steps are required to produce smooth uniform surfaces. However, it is found that there can be a build up of stress near the surface of the wafer, largely as a consequence of the mechanical abrasion steps. A stress imbalance between a front and rear of the wafer can result in wafer bow, which in turn, can have adverse consequences for future process steps. To minimise the stress imbalance, various methods can be employed, such as wet chemical etching, chemical mechanical polishing or plasma etching, or a combination of these processes.
When using plasma etching to relieve stress or to produce a surface texture on a wafer substrate the key requirements are the uniform removal of material from the surface, without producing wafer damage, at as high a rate as possible. Inherent in this plasma etch process is a change in stress in the wafer that can result in wafer deformation. Excessive deformation can result in wafer breakage which is very undesirable, since the etch chamber must then typically be vented to remove the debris. This is turn results in costly downtime for the plasma etch tool.
In conventional plasma etching, the substrate is retained in position using mechanical or electrostatic clamps, and cooled through the use of a coolant, such as helium, for example. However, when the entire surface of the wafer needs to be etched, mechanical clamping cannot be used as the areas of the wafer where the clamp makes wafer contact, and thus covers the wafer, will receive a different etch rate to the exposed or uncovered areas of the wafer. Furthermore, when the substrate is an insulator such as sapphire or glass for example, an electrostatic clamp or chuck cannot be used.